1. Field of the Invention
The present invention relates generally to integrated regulator technology, and more specifically to an integrated regulator having an adjustable reset threshold.
2. Description of the Prior Art
A wide variety of situations require that a desired voltage value be maintained or supplied to a given system or application. Often, this voltage value must be within a strictly defined tolerance of the desired voltage value. A case in point is a microprocessor-based system where it is key that voltages supplied to the microprocessor be within rigidly defined limits in order to avoid false triggering of data to the microprocessor.
Referring to FIG. 1, a schematic diagram of an integrated regulator, according to the prior art, is shown. As indicated by the dashed lines of FIG. 1, the elements of the integrated regulator are contained within an integrated circuit device. The elements of the integrated regulator 10 of FIG. 1 include a Transistor 20, an Op Amp 22 (operational amplifier), a comparator 24, a Voltage Reference block 26, and resistive elements, resistors R3, R4, R5 and R6. Voltage is supplied to the integrated regulator via the Voltage In pad 12 to transistor 20, the voltage output signal 15 of the integrated regulator, Vout, is available through the Voltage Out pad 14, and the Reset Out signal 25 is available through the Reset Out pad 18. The logic state of Reset Out signal 25 on pad 18 is produced by comparator 24 and is indicative of whether voltage output signal 25 has violated the reset threshold of the integrated regulator. The reset threshold of integrated regulator 10 is the trip point at which a given value of voltage output signal 15 will cause the Reset Out signal 25 on pad 18 to change from a first logic state to a second logic state.
The goal of the integrated regulator is to produce a voltage output signal 15, Vout, which matches, as much as possible, the voltage of the Voltage Reference block 26, and Op Amp 22 operates to regulate voltage output signal Vout 15. The Op Amp has two input signals: the signal at Node 2, determined by the value of the resistive elements, which is provided to the negative input terminal of the Op Amp 22 and the voltage signal from the Voltage Reference block 26 which is provided to the positive input terminal of Op Amp 22. Voltage output signal Vout 15 may be monitored to determine whether it remains within an accepted tolerance of its desired value. The value of voltage output signal Vout is given by the following equation: ##EQU1## where Vref is equal to the voltage signal produced by the Voltage Reference block and supplied to the positive input terminal of the Op Amp.
While Op Amp 22 actually performs regulation of voltage output signal 15, comparator 24 senses whether voltage output signal 15 is being properly regulated, and if it is not, communicates this information via the Reset Out signal 25 on pad 18. Thus the logic state (either active or inactive) of Reset Out signal 25 on pad 18 indicates whether voltage output signal 15 has crossed the trip point of the regulated integrator, called the reset threshold. If Op Amp 22 is not regulating properly, such as when voltage output signal 15 is lower than the reset threshold of integrated regulator 10, the reset threshold, referred to as RESET.sub.OFF, is given by the following equation: ##EQU2## Shorting out resistive element R5 is shorted out then the equation becomes: ##EQU3## Resistive element R5 must have a value greater than 0 Ohms to introduce hysteresis to comparator 24.
As an example, customer using the integrated regulator in a microprocessor system may require that the desired value of voltage output signal Vout be 5 volts and further that the voltage output signal Vout may only deviate from 5 volts by .+-.0.2 volts; otherwise, a Vout value not within this tolerance range may cause the microprocessor to falsely trigger incorrect data states. Thus, in this example, voltage output signal Vout is within acceptable limits only so long as its value remains in the range from 4.8 volts to 5.2 volts; the trip points, then of integrated regulator 10, are 4.8 volts and 5.2 volts. When Op Amp 22 has not been successful at regulation, voltage output signal Vout 15 becomes an unacceptable value, i.e. less than 4.8 volts or greater than 5.2 volts, and it may be brought back within the accepted tolerance by making internal adjustments to resistive elements R3, R4, R5 and R6. When voltage output signal 15 has passed the reset threshold trip point, then Reset Out signal 25 produced by comparator 24 becomes active (a logic low voltage level on pad 18) and thus communicates to the user that voltage output signal 15 is out of regulation.
While the integrated regulator 10 of FIG. 1 has Op Amp 22 for regulating voltage output signal 15 and comparator 24 for sensing whether Op Amp 22 is keeping voltage output signal 15 within regulation, a major shortcoming of integrated regulator 10 is that it does not provide a way for easily and readily adjusting the reset threshold, or trip point, as desired or when necessary. Thus a user of integrated circuit 10, perhaps a customer of a semiconductor manufacturer, must make do the reset threshold of the integrated regulator chosen by the manufacturer since the reset threshold of the device is not easily and readily adjustable.